Exhaust noises of an internal combustion engine for vehicles, such as motorcycles, are caused by pressure waves generated by the difference between the pressure in the cylinder and that in the exhaust pipe at the moment when the exhaust valve is opened, resonant pipe sounds amplified and generated when the pressure waves reciprocate within the pipe, and gas current noises generated by the current of the exhaust gas.
The pattern of these exhaust noises is shown in FIG. 11 wherein the abscissa represents the frequency distribution in Hz, and the ordinate represents the exhaust sound level in dB. From the line A showing the actually measured values, it is seen that considerably high level exhaust noises are generated in a range from a low frequency to a high frequency.
Various types of mufflers have already been proposed and practiced for eliminating exhaust noises, viz., an expansion type muffler, resonance type muffler, sound absorption type muffler, and muffler of a type combining either two or three of these various mufflers. However, in the conventional muffler, the more complicated the structure becomes, the more gas current sounds are generated.
The above mentioned phenomenon is pronounced in the medium and high sound ranges. For example, in the expansion type muffler, the silencing effect is obtained, as shown by the line B in FIG. 11, in a range from a comparatively low sound to a partly medium sound. However, the silencing effect in a medium sound range to a high sound range is so low that there is no silencing effect at a high frequency. Therefore, the medium and high frequency components are discharged as noises of the exhaust system.
As a countermeasure to the above problem, a practically ideal silencing effect can be obtained by noting the characteristic of the expansion type muffler, i.e., the fact that it is effective to a low sound range, and combining it with means having a silencing effect in the medium and high sound ranges with the characteristic of this silencing means being predetermined as line C in FIG. 11, so that both characteristics are combined. On this basis, a means for increasing the silencing effect in the medium and high sound ranges is proposed. It is essential to increase the overlapped area and interfering area of the characteristic B of the expansion type muffler and the characteristic C of the silencing means in the medium and high sound ranges, and to improve the sound reducing effect of the silencing means.
Therefore, it is intended to reduce and silence exhaust noises as a whole by providing a sound absorbing type tail pipe in the tail part of an exhaust pipe, silencing the medium and high sound ranges with this tail pipe, and combining this with the expansion type muffler. However, it is difficult to obtain a sufficient silencing effect with a conventional sound absorbing type silencing tail pipe.
It is known that, in the conventional sound absorbing type silencing tail pipe, many perforations are provided on the peripheral wall of a part of the exhaust pipe assembled in the muffler. Sound absorbing material, such as glass wool, is mounted by winding or so on the outer periphery of the perforated part so that the medium and high frequency components propagated as longitudinal waves within the exhaust pipe may be absorbed by the sound absorbing material disposed on the perforations, and thus reduce the exhaust noise level.
However, because the conventional sound absorption type silencing tail pipe is of such structure as mentioned above, with the increase of the exhaust current velocity, the sound waves will pass through the pipe without touching the sound absorbing material outside the wall of the exhaust pipe and will be charged out into the atmosphere. Further, because the sound waves will pass through the outlet pipe, the current velocity energy of the exhaust will be discharged without being reduced. As a result, jet sounds will be generated at the open end of the tail pipe. Thus, a sufficient silencing effect can hardly be expected from the conventional sound absorption type silencing tail pipe.
This relation is shown in FIG. 10 wherein the abscissa represents the frequency in Hz, and the ordinate represents the sound attenuation in dB. The characteristic of a conventional sound absorption type silencing tail pipe is shown by line D. As line D shows, the frequency level adapted to silencing is high and the sound attenuation is low. Therefore, even if lines B and C in FIG. 11 are combined, there would be no range matching the characteristic of the expansion type muffler; the sound reduction would be low; silencing of the medium and high frequency ranges as a whole would not be sufficiently made; and no effective silencing would be expected in the entire range of exhaust noises.
In view of the foregoing, it can be said that today a muffler having good silencing effect in the entire range of exhaust noises has not yet been proposed, and such a muffler is desired.
However, when the above mentioned sound absorption type silencing tail pipe is combined with an expansion type muffler as described above, the silencing effect as a whole is improved. In the expansion type muffler, an expansion chamber of a volume larger than that of the exhaust pipe is interposed in the exhaust system; the exhaust is introduced into the expansion chamber; the sound is attenuated in the chamber; and good silencing effect in a range from the low sound range to partly medium sound range is obtained as described before. In this type of muffler, the interior of the expansion chamber is sectioned with a partition plate, and the sectioned chambers are provided in stages to increase the attenuating effect. It is known that, in such type of muffler, when the length of the communicating pipe communicating with the interior of the expansion chamber with the next stage from the expansion chamber and further with the outside atmosphere is made long, and when the volume of the expansion chamber is also predetermined properly, the silencing effect will be improved. However, because the muffler has been made smaller taking the performance into consideration from the viewpoint of the assembling space and appearance, the length of the communicating pipe cannot readily be made long enough.
Also, sound absorbing material is generally attached to the inner wall of the muffler. However, it is necessary to consider the fact that when the area of the sound absorbing material is as large as possible, the sound absorbing effect is improved. Further, because welding traces are exposed outside the muffler or the like, particularly in a motorcycle, as the exhaust system is exposed, it is desirable that weldings be minimized. In spite of these requirements, it is desirable that the sound absorbing material assembling structure and fitting work be simplified.